The present invention generally pertains to a method of displaying colorized two-dimensional medical images of human anatomy. Specifically, the present invention pertains to a method of assigning predetermined range of colors to computed tomography images for enhanced visualization.
Medical imaging systems, such as Computed Tomography (CAT, or CT) scans, are well known in the art. CT imaging systems produce anatomical images by projecting x-ray beams through the desired portion of the human body. The x-ray beams undergo attenuation proportional to the densities of organs through which they travel. Detectors are arranged to receive the attenuated beams, and two-dimensional cross-section ‘slices’ are constructed by a computer, based on relative radiological densities of the internal organs and tissues.
CT densities are defined in Hounsfield Units (HU), named after the Nobel Prize winner who developed x-ray computed tomography. HU are units derived from exponents of attenuation ratios that range from −1024 to 4096 with values of −1000 for air to 0 for water, and to over 1000 for compact bones. Traditionally, CT scan images have been constructed using grayscale such that ‘white’ represents particles with the highest densities and ‘black’ represents particles with the lowest densities. Since HU may take on over 5000 distinct values, computed images must be able to incorporate over 5000 distinct shades of gray in order to accommodate the full density spectrum. This presents a serious problem in interpreting computed images, because human eyes cannot discern over 5000 distinct shades of gray. For example, an abdominal CT scan may contain dense spinal bones, which would appear ‘white’, surrounded by much less dense abdominal soft tissues, which would collectively appear ‘blackish’ or ‘grayish.’ With range of 5000 shades of gray, various abdominal soft tissues that share narrow range of HU values may not be distinguishable to human eyes.
In order to remedy this problem, prior art CT imaging systems utilize user-defined ‘viewing window’ which sets upper and lower HU threshold values displayable on the display screen. FIG. 1 shows a thoracic CT scan image with ‘abdominal windows’ having a window with width of 400 HU and center of 30 HU. With this setting, tissues that have Houndsfield density between 230 HU and −170 HU (range of 200 HU above and below a center value of 30) will be displayed in shades of gray. Setting the displayable threshold limits to ‘400 HU width/30 HU center’ allows any structures with densities of 230 HU and above to appear white, and any structures with densities of −170 HU or below to appear black. Structures with densities that fall between 230 HU to −170 HU are displayed in shades of gray proportional to their relative densities within the displayable threshold limits. Thus various soft tissues that share narrower HU values may become more easily discernible in this viewing window.
Upper and lower displayable thresholds may be changed by the user (i.e. radiologist) to target specific organs or tissues for display. FIG. 2 shows the ‘lung windows’ with width of 1500 and center of −700 (HU threshold limits 50 to −1024), and FIG. 3 shows the ‘bone windows’ with width of 2000 and center of 350 (HU threshold limits 1350 to −650). As such, inspection and examination of plurality of organs and soft tissues may require plurality of viewing windows, each with different upper and lower displayable threshold values. Since multiple viewing windows must be established for each CT scan slice, radiological diagnoses which involve examination of plurality of internal organs are difficult and time consuming.